1. Field of the Invention
The disclosure is related to a method for preparing aluminum nitride, and more particularly, to a convenient and efficient method for preparing aluminum nitride.
2. Descriptions of the Related Art
Aluminum nitride has been widely used for electronics in recent years because it has high thermal conductivity, (normally, the thermal conductivity of commercial aluminum nitride is about 170˜230 W/m/K, which is close to that of silicon carbide and beryllium oxide, and is 5 to 7 times of aluminum oxide), low dielectric constant and dielectric loss, good electric insulating quality, and low thermal expansion coefficient, which is close to that of silicon (4.2×10−6/T) and gallium arsenide (5.7×10−6/° C.). Aluminum nitride is also not toxic as beryllium oxide, and it is usually prepared with reduced manufacturing cost. For example, aluminum nitride has its applications in semiconductors and microelectronic packaging substrates, chip carrier substrates of high brightness LED, electronics and lighting devices of automobile, heat dissipating materials of high power electronic device and so forth, and it has great potential to replace other ceramic substrate materials.
General methods to prepare aluminum nitride include gas phase reaction method, organo-metallic precursor method, alumina powder carbon reduction nitriding method, aluminum powder direct nitriding method and combustion synthesis method. The followings describe each the above-mentioned methods and the disadvantages thereof:
(1) Gas phase reaction: the reaction formula is AlCl3+4NH3→AlN+3NH4Cl. The reaction temperature is 900 K to 1500 K, the required reaction time is longer than 5 hours, and the product type includes crystalline and amorphous aluminum nitride powder. Generally, the conversion rate of the product is about 80%. The disadvantage is that the production cost is high and the yield remains desired, rendering those methods unsuitable for industrial production.
(2) Organo-metallic precursor method: R3Al+NH3→R3AlNH3, R3AlNH3→R2AlNH2+RH, R2AlNH2→RAlNH+RH, RAlNH→AlN+RH. The operating temperature is 400 K to 1000 K and the reaction time is 10 to 240 minutes. The product is aluminum nitride (AlN) and alkanes, which will decompose and produce carbon deposition during the reaction, so that the product need to place in the air and to heat to remove carbon is necessary. This step may lead to higher oxygen content. In addition, this method is also costly and not productive so that it is not suitable for industrial production either.
(3) Alumina powder carbon reduction nitriding method: Al2O3+N2+3C→2AlN+3CO. The operating temperature is 1500 K to 2200 K and the reaction time is 5 hours or more.
(4) Aluminum Powder Direct Nitriding Method:
(a) 2Al+N2→2AlN, aluminum powder direct nitriding method is to heat aluminum powder in nitrogen, and directly transform aluminum powder into aluminum nitride powder through nitriding reaction; Al and N begin to react at 500° C., and during 500˜600° C., the surface oxide film on aluminum particle is removed by forming of volatile sub-oxide by reactions. However, the gradual formed nitride film on the particle surface will hinder further permeation of nitrogen and slow down the nitriding process, such that it needs a secondary nitriding process in order to improve the nitriding efficiency. Specifically, the primary nitriding is to perform at 800° C. for one hour, and then the product goes through a secondary nitriding at 1200° C. after ball-milling process to obtain uniform aluminum nitride powders.
(b) The reaction products of aluminum powder direct nitriding method often require both multiple pulverization and nitriding treatment, therefore the production cycle prolongs and the production cost increases. Moreover, the introduction of impurities during the ball-milling process negatively affects the purity of aluminum nitride powder. Therefore, it is difficult to prepare aluminum nitride powder with high purity and fine grain size by direct nitriding method, which fails to meet the requirement of raw material powder for obtaining qualified aluminum nitride ceramics.
(5) Self-Propagating High-Temperature Synthesis (SHS):
(a) This method is to design reactants of exothermic reactions. The reaction is firstly to initiate an external source of heat, and then it is self-maintaining by the release of heat itself. This method also forms combustion wave that propagates and spreads. Combustion synthesis method (or high-temperature self-propagating method) is a new type of inorganic refractory material preparation technology that occurred in the Soviet Union during 1960s. Because of its advantages of short synthesis time and low energy consumption in refractory material synthesis, it is increasingly capturing much attention from materials academia.
(b) This method has been used domestically and abroad to study the synthesis of aluminum nitride powder. Aluminum powder is to ignite by external heat under high pressure, the reaction between Al and N generates high chemical reaction heat that sustains the reaction itself until Al powder is complete convert to AlN. Preparing aluminum nitride powder through self-propagating high-temperature synthesis method is substantially using direct nitriding aluminum, as indicated by the reaction formula of 2Al+N2→2AlN. The aluminum nitride powder prepared by this method, however, does not need to be exposed to 1000° C. for a long time of nitriding as in the direct nitriding method. It does not require any external heat source except for igniting, and therefore less energy is to consume, which may result in lowered cost and higher production efficiency.
(c) In SHS process using the Al—N2 system, since the melting point of aluminum is low, as in the direct nitriding method, the molten aluminum tends to agglomerate at the high temperature of the combustion synthesis reaction, which hinders nitrogen from penetrating into the powder and makes it difficult for aluminum powder to be completely nitrided.
(d) Therefore, the general disadvantages of the combustion synthesis method are:
(d1) The conversion rate is not high enough.
(d2) It is not easy to have mass production. That is, during mass production, the conversion rate will decline.
(d3) The product is in the state of melting agglomerate or sintering, which makes it hard to be grinded. In other words, grinding the product prepared using the conventional methods above is time-consuming and energy consuming. Wearing off grinding balls also causes increase in the impurity content in powders, which problem shares all of the existing combustion synthesis methods. There are studies showing that adding a certain amount of aluminum nitride as diluents to aluminum powder can prevent molten aluminum from agglomeration and can improve the permeability of nitrogen. Even so, however, this approach will complicate the process and increase the cost.
As described above, the conventional combustion synthesis method has the problem of melting agglomeration. In order to solve this problem, it is necessary to carry out ball grinding for multiple times, which in turn at the same time would introduce more impurities. The present disclosure discloses pulverizing aluminum nitride produced in high reaction temperature through a high-temperature jet-mill. Thus, reaction and pulverizing aluminum nitride layer can happen at the same time. After pulverizing aluminum nitride layer, it is possible to solve the problem that nitrogen cannot penetrate into the powder so that aluminum powder cannot be complete nitride.